The present invention relates to a tool for measuring the mechanical properties of a ground formation, typically an underground formation traversed by a borehole such as a hydrocarbon well.
When drilling a well such as a hydrocarbon well, it is necessary to obtain information about the nature of the formation being drilled. While some information can be derived from the drilled material returned to the surface, it is often necessary that measurements be made in situ or on larger samples in order to obtain the necessary information. Certain properties can be measured by lowering a tool into the well and making non-intrusive measurements while the tool is moved vertically. This technique is known as electrical logging. The measurements made by the tool are returned to the surface as signals in a wire cable where they can be detected and analysed. Consequently, the technique is also known as wireline logging. Commonly measured properties relate to inherent properties of the formation such as electromagnetic, nuclear and sonic behaviour of the formation and allow the determination of formation resistivity, natural gamma-ray emission and sonic wave speed. However, wireline logging has not been particularly successful to date in determining mechanical properties of formations since this generally involves destructive testing of a sample. The approaches which have been used previously are either the immobilisation of a tool within the wellbore to allow in situ testing or side-coring to retrieve a sample of rock which is returned to the surface for laboratory testing. This latter approach is expensive and time consuming and neither technique allows a continuous logging approach in which measurements are made continuously as the tool is moved through the borehole.
It is an object of the present invention to provide a tool which can provide mechanical properties of the formations traversed by a borehole in a continuous logging operation.